Flexible liquid storage tanks of relatively high capacity that exhibit a pillow- or sausage-like shape when filled are widely known as “pillow tanks.” They are typically composed of thermoplastic materials, such as polyether or polyester, and may include two or more layers of material. These tanks can be used to store, for example, gasoline, diesel fuel, jet fuel, potable water or hazardous liquid waste. Flexible storage tanks have the advantages of light weight and portability. Also, flexible storage tanks can be stored in a relatively small volume until needed.
However, conventional flexible storage tanks typically include seams, which are often the source of leakage. In particular, conventional flexible storage tanks are constructed in shapes that subject the seams of their flexible walls to stresses oriented perpendicularly to the flexible walls. These perpendicular stresses, widely known as “normal stresses,” are more difficult to seal against than “shearing stresses” (also known as “tangential stresses”).
For example, U.S. Pat. No. 3,453,164, issued to Gursky et al., describes a method of building fabric elastomeric containers in which a fabric is cut into strips and a tube is assembled by overlapping the edges of the strips in stitching to form individual seams. A liquid polyurethane reaction mixture and a material suitable for forming a fuel vapor barrier are applied to both sides of the assembled tube. Two end members are formed by folding pieces of the fabric into U-shapes. One of the U-shaped end members is cemented on each end of the tube to form a substantially rectangular container. Then each of the corners is trimmed to remove a triangle of fabric from each corner. Specially shaped and sized pieces of knit fabric are cemented over the trimmed areas to produce somewhat rounded corners, as indicated for knit fabric piece 41 in FIG. 4 of the Gursky et al. patent. However, as can be seen in FIG. 5 of the Gursky et al. patent, the finished container is still substantially rectangular in shape. Because neither this substantially rectangular shape nor the shape of fabric piece 41 matches the configuration of hydraulic forces within the container when it is filled with a liquid, at least some of the seams in the container of Gursky et al. patent are likely to be exposed to predominantly normal stresses.
A collapsible storage tank is described in U.S. Pat. No. 4,573,508, issued to Knaus, as including a substantially rectangular tank body composed of vulcanized inner and outer envelope structures. As can be seen in FIG. 3 of the Knaus patent, the collapsible storage tank includes rectangular corners and a peripheral seam that do not conform to the configuration of hydraulic forces that arise when the collapsible storage tank is filled with a liquid.
U.S. Pat. No. 3,919,030, issued to Jones, describes an elastic, fluid impervious storage tank having an intermediate section and a pair of end sections. Each of the end sections is reportedly formed from a single blank composed of a fiber-reinforced elastomer, which is cut and folded so that the corner portions are of rounded or arcuate configuration. The Jones et al. patent recites that this rounded corner portion greatly increases the strength of the storage tank, as compared to tanks having angular corner portions. According to the Jones patent, the cuts in the end section are closed by adhering a pre-formed arcuate inner attachment member 36 to the inside surface of the corner portion and, also adhering a pre-formed arcuate outer attachment 37 to the outer surface of each corner portion. The need to employ pre-formed members 36, 37 is a disadvantage in some situations. Also, it appears that any advantages associated with the storage tank of the Jones patent are limited to storage tanks that are small enough for the end sections to be constructed from one or two blanks of fabric material. Relatively larger tanks, which must necessarily be constructed of a number of fabric panels, are excluded.
A need exists for an improved flexible storage tank constructed in a configuration that is less susceptible to leaking when constructed of commonly available materials. Preferably, the improved tank is formed in shapes of revolution having relatively greater radii, as compared to conventional flexible storage tanks. Ideally, the seams of the improved tank are substantially under shearing stress, rather than normal stress, when the tank is filled with a liquid.